This study is designed to analyze the processes which underly the onset and prevention of smoking and other behaviors detrimental to health. Approximately 2,000 young adolescents from ten schools serving a wide variety of populations are being studied longitudinally to measure the onset of smoking, alcohol and drug abuse and associated variables. A theoretical model predicting that environmental factors are mediated by specific beliefs and expectancies in interaction with psychological factors is being tested as an explanation of onset. From this model a broad approach to prevention has been derived. The ten schools are formed into five matched pairs and one is randomly assigned from each pair to receive the preventive intervention. Rates of onset and underlying processes will be compared in each pair. Physiological measures (saliva thiocyanate) are being employed to assess smoking.